The invention relates to the general field of lateral diffused MOS (LDMOS) devices with particular reference to increasing the punch-through voltage.
An LDMOS device (Lateral Diffusion Metal Oxide Semiconductor) is basically a MOSFET fabricated using a double diffusion process with coplanar drain and source regions. The present invention is concerned with the particular case of P channel devices. In general, a P-type LDMOS device is difficult to integrate with processes for manufacturing N-type LDMOS devices without adding an extra mask. The main problem is that the P-type LDMOS cannot achieve the same breakdown voltage as its N-type counterpart because of early punch-through.
An N-epitaxial layer is always selected for the formation of LDMOS devices since the N-epi can be used as the drift region of the MOS drain to sustain high voltage. However the drain of a P-type LDMOS is formed by a P-implant process so a P-drain/N-epi/P-substrate structure is formed. When a high negative voltage is applied to the drain of a P-type LDMOS, punch-through to the P-substrate can occur very early.
This is illustrated in FIG. 1 which shows a typical structure of the prior art. Nxe2x88x92 body of silicon 12 (that typically has a resistivity between about 0.1 and 1 ohm-cm) is isolated from neighboring devices by P+ boundaries 13. Pxe2x88x92 well 18 extends downwards from the top surface and includes P+ drain 17b which is positioned to lie between two areas 14 of field oxide. Source 17a lies well outside Pxe2x88x92 well 18, also between two areas 14 of field oxide. Adjacent to the source is N+ area 19 to which is shorted thereto through metallic contact 15. Area 19 serves to provide bulk contact to Nxe2x88x92 body 12, providing it with a voltage bias.
The distance L seen in FIG. 1 defines the channel since it lies beneath polysilicon gate 16. There is also a layer of gate oxide beneath gate 16 which is not explicitly shown in this figure. It will be noted that L does not extend all the way to the boundary between regions 12 and 18. This is because, with the application of negative voltage Vd to the drain, Pxe2x88x92 depletion region 10 extends outwards, effectively enlarging region 18, so P channel L does not have to extend all the way to the original region 18.
Thus the formation of depletion region 10 serves to reduce the on-resistance of the device. The down side of this, however, is that, with the application of relatively low drain voltage, depletion region 10 becomes thick enough to touch Pxe2x88x92 substrate 11 and punch-through occurs. This effect is illustrated in FIG. 2 which is a plot of drain current vs. drain voltage for a device of the type illustrated in FIG. 1. As can be seen, punch-through has occurred at about 10 volts, at which point the drain current is no longer controlled by the gate voltage. The present invention discloses how this problem may be overcome while continuing to retain compatibility with the simultaneous manufacture of N-channel devices and, particularly, without the need to introduce an additional mask into the manufacturing process.
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Pat. No. 5,517,046, Hsing et al. disclose an N-channel LDMOS device with a 2 step doping Nxe2x88x92 and N+ in an epi layer. Their process and their structure differ from the present invention""s process, theirs being a P channel LDMOS device whereas the present invention discloses an N channel device. As a consequence, the N+ buried layer that they teach, while improving on-resistance, has no significant effect on the breakdown voltage.
Other examples of LDMOS devices can be found in U.S. Pat. No. 5,940,700 (Galbiati et al.), U.S. Pat. No. 6,046,473 (Blanchard), and U.S. Pat. No. 6,069,034 (Gregory).
It has been an object of the present invention to provide a P-type LDMOS device having significantly higher punch-through voltage than similar devices of the prior art.
Another object has been to provide a process for manufacturing said device, said process being fully compatible with the manufacture of a N-type LDMOS device without requiring use of an additional mask.
These objects have been achieved by preceding the epitaxial deposition of Nxe2x88x92silicon onto the Pxe2x88x92substrate with an additional process step in which a buried N+ layer is formed at the surface of the substrate by ion implantation. This N+ buried layer significantly reduces the width of the depletion layer that extends outwards from the Pxe2x88x92 well when voltage is applied to the drain thus substantially raising the punch-through voltage.